湖南白石峰岩体的年代学、地球化学特征及其构造背景

Geochronology, Geochemical Characteristics and Tectonic Significance of the Baishifeng Pluton in Hunan

摘要: 白石峰岩体位于湖南衡山，是衡山复式岩体的组成部分，侵入到新元古代板溪群及泥盆—二叠纪地层中，岩性为二云母花岗岩。LA-ICP-MS锆石U-Pb年龄为157 ± 2 Ma与151 ± 2 Ma。白石峰岩体为强过铝质(A/CNK > 1.1)、镁质、高钾钙碱性系列花岗岩。微量元素含量较低(∑REE = 50.4 × 10−6~197.6 × 10−6)，轻稀土富集，Ba、Nb、Sr和Ti相对亏损，表现出明显的负Eu异常(δ Eu为0.32~0.62)。岩体的εHf(t)值分别在−13.14~−11.06与−18.76~+0.51之间，显示源区为壳源。综合分析表明，白石峰花岗岩形成时代为晚侏罗世，花岗岩类型属于S型花岗岩，可能为前寒武纪基底部分熔融形成。白石峰花岗岩形成于后碰撞环境，与华南地区晚中生代整体呈伸展的构造环境有关。

Abstract: The Baishifeng pluton, located in Hengshan, Hunan Province, is a component of the Hengshan complex. On the outcrops, the pluton invades into the Neoproterozoic Banxi Group and the Devonian-Permian strata. Petrographically, the pluton displays the characteristics of the two-mica granite. Two samples are analyzed by LA-ICP MS and yield the weighted mean 206Pb/238U age of 157 ± 2 Ma and 151 ± 2 Ma, respectively, suggesting Late Jurassic magmatic emplacement. The Baishifeng pluton is characterized by a strongly peraluminous (A/CNK > 1.1), magnesia, high-potassium calc-alkaline series. The content of trace elements is low (∑REE = 50.4 × 10−6 - 197.6 × 10−6), but shows high LREE with obvious negative Eu anomalies (δ Eu is 0.32 - 0.62). In the primitive mantle normalized spidergram, the samples exhibit relative depletion in Ba, Nb, Sr and Ti. The two samples have notably negative εHf(t) values (−13.14 - −11.06 and −18.76 - +0.51, respectively), indicating generation in crust. The granite samples show S-type granite affinity. Our results, combined with other geological evidence, indicate that the Baishifeng pluton may be formed by the melting of the Precambrian basement in the post-collision setting. We infer that the Baishifeng granitic pluton is more likely to be related to the overall extensional setting during Late Mesozoic in South China.